Milled carbon fiber and process for producing the same

ABSTRACT

Milled carbon fibers are provided which have a fiber cut surface and a fiber axis intersecting with each other at cross angles, the smaller one thereof being at least 65° on the average. The milled carbon fibers may have a specific surface area as measured by the BET method of 0.2 to 10 m 2 /g. The milled carbon fibers may be obtained by a process comprising melt spinning of mesophase pitch, infusibilization, milling of the infusibilized pitch fibers as obtained or after a primary heat treatment at low temperatures in an inert gas and a high-temperature heat treatment in an inert gas. Even when the graphite layer plane has achieved high-level growth, the above milled carbon fibers have low reactivity with a metal of high temperature or the like during the molding or use thereof because the proportion of reactive exposed surface of the inner portion of the fiber is small, so that the use of the milled carbon fibers can improve the mechanical strength and high-temperature heat resistance of the composite material.

FIELD OF THE INVENTION

[0001] The present invention relates to milled carbon fibers. Moreparticularly, the present invention is concerned with milled carbonfibers which have a large surface area available for contact withmetals, etc., so that it is suitable for improving the rigidity andhigh-temperature heat resistance of metals, alloys and the like, therebyensuring advantageous utilization thereof in, for example,carbon-fiber-reinforced composite materials. Also, the present inventionis concerned with a process for producing the milled carbon fiber.

BACKGROUND OF THE INVENTION

[0002] The carbon fiber is lightweight and has high strength andrigidity, so that in recent years it is utilized in a wide spectrum offields from the aerospace and aircraft industry to the generalindustries.

[0003] For example, carbon-fiber-reinforced plastics are actually widelyutilized as structural materials having high specific strength andspecific modulus of elasticity. Further, carbon-fiber-reinforced metals(CFRM), such as carbon-fiber-reinforced aluminum alloys andcarbon-fiber-reinforced magnesium alloys (hereinafter referred to as“CFRAl(Mg)”), have been developed as materials having excellenthigh-temperature dimensional stability and thermal deformationresistance, and their use is anticipated as a material for use instructural members for aerospace and aircraft and engine members forvehicles.

[0004] However, the production of CFRAl(Mg) has encountered, forexample, a problem such that not only is the wettability of the carbonfiber with molten Al (or Mg) poor but also, once the wetting iseffected, the carbon fiber reacts with Al to thereby form Al₄C₃ with theresult that the strength of the material is lowered.

[0005] The amount of formed Al₄C₃ is connected with the type of thecarbon fiber. That is, the carbon fiber produced by heat treating at atemperature of about 2000° C., known as “graphitized carbon fiber”, hasa high carbon crystallization degree and a strong carbon-to-carbon bondto render itself stable, as compared with the carbon fiber produced byheat treating at a temperature of about 1500° C., known as “carbonizedcarbon fiber”, so that the reactivity with molten Al alloy or the likeis poor, thereby minimizing the formation of carbides, such as aluminumcarbide.

[0006] Therefore, the mechanical properties of the CFRAl(Mg) aresuperior when the graphitized carbon fiber is used as reinforcement.

[0007] The graphite crystals of the graphitized carbon fiber aregenerally highly anisotropic from the dynamical, electrical andscientific viewpoints, because the carbons interact each other betweenthe graphite layer planes with only weak intermolecular force while thesp² carbons are strongly bonded within each of the graphite layer planes(c-planes).

[0008] In the so-called monoaxially oriented structure in which thec-planes are arranged parallel to the fiber axis, there may be somemutually different microstructures or high-order structures, dependingon the type of the carbon fiber precursor [polyacrylonitrile (PAn),rayon, pitch, etc.].

[0009] Of the above precursors, when mesophase pitch with greatergraphitizability is used as a starting material, the graphitization ismore readily promoted even at the same heat treating temperature tothereby produce carbon fibers having higher modulus of elasticity.Therefore, the use of carbon fibers of high elastic modulus derived frommesophase pitch is especially promising in the formation of a compositewith an aluminum alloy and the like.

[0010] On the other hand, from the viewpoint of moldability, the use ofmilled carbon fibers is advantageous in respect of the degree of freedomof molding and molding/working costs, although the molding with the useof lengthy carbon fibers is suitable for producing a fiber-reinforcedmetal composite having excellent mechanical properties.

[0011] The use of the milled carbon fibers in the fiber-reinforced metalcomposite leads to the increase of the surface area brought into contactwith metals. The opportunity of reaction with the metals becomes high asmuch as the above increase, so that greater attention must be paid tothe formation of carbides.

[0012] Coating with silicon carbide or precoating with a matrix metal,such as aluminum, at low temperatures has been tried for the purpose ofimproving the wettability with metals and suppressing the abovereaction.

[0013] However, these conventional trials have had a drawback in thatthe efficacy is low for the cost increase involved.

[0014] The inventors have made extensive and intensive studies with aview toward resolving the above problems. As a result, they have foundthat the configuration of the milled carbon fiber, especially themorphology of the surface thereof, has an important relationship withthe formation of carbides with metals, and that the reaction of themilled carbon fiber with metals can be suppressed by improving the aboveconfiguration. The present invention has been completed on the basis ofthe above findings.

OBJECT OF THE INVENTION

[0015] The present invention has been made with a view toward obviatingthe above drawbacks of the prior art. Thus, the object of the presentinvention is to provide milled carbon fibers which have desirably growngraphite layer planes and accordingly a low reactivity with metals, sothat it can provide a lightweight and rigid fiber-reinforced metalhaving excellent heat resistance at high temperatures, and also toprovide a process for producing the desired milled carbon fibers.

SUMMARY OF THE INVENTION

[0016] The milled carbon fibers of the present invention are oneproduced from mesophase pitch, which have a fiber cut surface and afiber axis intersecting with each other at cross angles, the smaller onethereof being at least 65° on the average.

[0017] The milled carbon fibers of the present invention preferably havea specific surface area as measured by the BET method of 0.2 to 10 m²/g.

[0018] The process for producing milled carbon fibers according to thepresent invention comprises the steps of:

[0019] melt spinning mesophase pitch to obtain pitch fibers;

[0020] infusibilizing the obtained pitch fibers;

[0021] milling the infusible pitch fibers as obtained or after a primaryheat treatment at 250 to 1500° C. in an nert gas; and

[0022] subjecting the obtained milled fibers to a high-temperature heattreatment at 1500° C. or higher in an inert gas.

BRIEF DESCRIPTION OF THE DRAWING

[0023] FIGURE is a schematic perspective of the milled carbon fiberprovided for explaining the cross angle of a fiber cut surface and afiber axis intersecting with each other.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention will now be illustrated.

[0025] The pitch as the starting material of the milled carbon fiberaccording to the present invention is optically anisotropic pitch, i.e.,mesophase pitch. The mesophase pitch can generally be produced frompetroleum, coke and other various raw materials. The mesophase pitch asthe starting material for use in the present invention is notparticularly limited as long as it is spinnable.

[0026] The desired mesophase-base carbon fiber produced by subjectingthe above starting pitch to spinning, infusibilization and carbonizationor graphitization according to the customary procedure permits freecontrol of the crystallization degree thereof.

[0027] The terminology “milled carbon fiber” used herein means a carbonfiber which is shorter than the carbon fiber of about 1 to 25 mmgenerally known as “chopped strand” and which has a length of about 1 mmor less.

[0028] The milled carbon fibers of the present invention have a fibercut surface and a fiber axis intersecting with each other at crossangles, the smaller one thereof being at least 65°, preferably at least70°, still preferably at least 75° on the average. The cross angle ofthe fiber cut surface and the fiber axis intersecting with each otherwill be illustrated below with reference to the appended FIGURE. Theappended FIGURE is a schematic perspective of an end portion of themilled carbon fiber provided for explaining the cross angle of the fibercut surface and the fiber axis of the carbon fiber intersecting witheach other. As illustrated, the carbon fiber 1 has a fiber cut surface(s) formed by the milling at an end portion thereof. In the presentinvention, the smaller angle (θ), on the average, of the cross angles ofthe fiber cut surface (s) and the fiber axis (d) of the carbon fiber 1intersecting with each other is used as the above value for numericallimitation.

[0029] Herein, the average of the cross angle (θ) is an average of thecross angles of at least 100 milled carbon fibers. In the calculation ofthe average of the cross angle (θ), when the carbon fiber has sufferedfrom longitudinal crack along the fiber axis (d) on the fiber cutsurface during milling, the cross angle (θ) is defined to be 0°. Theaverage of the cross angle (θ) of the fiber cut surface (s) and thefiber axis (d) intersecting with each other can be measured by the useof a scanning electron microscope (SEM).

[0030] The milled carbon fibers having an average of the cross angle (θ)of the fiber cut surface (s) and the fiber axis (α) intersecting witheach other which is at least 65° are cylindrical in the entireconfiguration thereof and have no sharply projecting portions such as anacicular portion from the fiber cut surface. That is, the milled carbonfiber of the present invention is cylindrical in the entireconfiguration thereof, and has a fiber cut surface nearly perpendicularto the fiber axis, in which the graphite layer has few sharpunevennesses inside.

[0031] The distribution of graphitization degree in the direction of theinner diameter of the cut surface of the carbon fiber produced from astarting pitch material is reported in G. Katagiri, H. Ishida and A.Ishitani, carbon 26, 565 (1988). This reference shows that the nearerthe surface the portion concerned, the greater the graphitization degreeand the higher the crystallization degree there. Also, as mentionedabove, it is preferred that the reinforcing carbon fiber for use in theCFRM be graphitized for reducing the formation of carbides due to thereaction with molten alloys. Therefore, in the carbon fiber derived frommesophase pitch, it is important that the carbon with lowcrystallization degree having originally been present inside the fiberis less exposed to the surface of the fiber during the milling.

[0032] On the other hand, the inventors' study and observation haverevealed that the angle of the cutting of the carbon fiber becomesnearly parallel to the fiber axis, depending on the force applied to thecarbon fiber during milling, so that the carbon fiber is cleaved alongthe graphite layer plane to thereby expose much of sharply unevengraphite layer plane present inside the fiber and, in extreme cases, torender the fiber acicular. The above average of the cross angle (θ) ofthis milled carbon fibers is less than 65°.

[0033] The above milled carbon fibers which are extremely marked in thearea of exposure of the graphite layer plane having originally beenpresent inside the carbon fiber, the above exposure resulting from thefrequent cleavages along the fiber axis and along the graphite layerplane during milling, that is, the milled carbon fibers whose average ofthe cross angle (θ) is less than 65°, are disadvantageous in molding andlong-time use at high temperatures. This is because, when thetemperature is high during the molding and use, the formation of carbidedue to the contact with the metal is extremely increased, therebygravely deteriorating the strength of the carbon-fiber-reinforced metal.

[0034] This strength deterioration would be attributed to an extremeincrease in the area of exposure of the reactive graphite layer planehaving originally been present inside the fiber, the above exposureresulting from cleavage along the fiber axis during the milling, whichincrease would cause the reaction between the metal and the carbon toproceed on the graphite layer plane.

[0035] For being suitable for use as metal fiber reinforcement, it ispreferred that the milled carbon fibers of the present invention have arelatively small specific surface area. Specifically, it is preferredthat the specific surface area as measured by the BET method be in therange of 0.2 to 10 m²/g, especially 0.3 to 7 m²/g. The specific surfacearea of the milled carbon fibers is measured in accordance with the BETone-point method in sorption and desorption of nitrogen gas at arelative pressure of 0.3.

[0036] When the above specific surface area is less than 0.2 m²/g, thewettability of the milled carbon fibers with a metal is likely todecrease so as for bubbles to remain between the fibers and the metalduring the molding, thereby deteriorating the strength properties of thecarbon-fiber-reinforced metal.

[0037] On the other hand, when the above specific surface area exceeds10 m²/g, the surface area brought into contact with the metal is likelyto be extremely high so as to increase the opportunity of carbideformation, thereby lowering the strength of the carbon-fiber-reinforcedmetal.

[0038] The milled carbon fibers of the present invention have beendescribed, and, hereinafter, the process for producing the milled carbonfibers will be described.

[0039] The process for producing the milled carbon fibers of the presentinvention is not particularly limited as long as the value of the crossangle of the fiber cut surface and the fiber axis intersecting with eachother is as described above and as, preferably, the value of thespecific surface area as measured by the BET method is also as describedabove.

[0040] The above process, for example, comprises spinning the abovemesophase pitch to obtain pitch fibers, infusibilizing the pitch fibers,milling the obtained infusible pitch fibers and effectingcarbonization/graphitization of the milled fibers.

[0041] The pitch fiber may be spun by any of the conventional melt,centrifugal, vortex and other spinning techniques. Especially, the meltblow spinning technique is preferred, collectively taking into accountthe production costs including spinning apparatus construction andoperating costs and the quality control including the degree of freedomin controlling fiber diameters.

[0042] The thus obtained pitch fiber is infusibilized by theconventional method. Although this infusibilization can be effected byheating in an oxidative atmosphere of air, oxygen, nitrogen dioxide orthe like or treating in an oxidative solution of nitric acid, chromicacid or the like, practically, it is preferred that the infusibilizationbe performed by heating in air at temperatures ranging from 150 to 350°C. in which the heating temperature is elevated at a heat-up rate of 3to 10° C./min.

[0043] The infusibilized pitch fiber may directly be milled andsubjected to high-temperature heat treatment forcarbonization/graphitization. Alternatively, it may first be subjectedto primary heat treatment at lower temperatures, and then milled andsubjected to the high-temperature heat treatment.

[0044] The milling of the infusibilized pitch fiber or the primarilyheat-treated carbon fiber may be performed by a procedure comprisingrevolving a rotor equipped with a blade at a high speed and contactingthe fiber with the blade to thereby cut the fiber in the directionperpendicular to the fiber axis. In this procedure, the milling may beperformed by the use of, for example, the Victory mill, jet mill orcross flow mill. In the above procedure, the length of the milled pitch(or carbon) fiber can be controlled by regulating the rotating speed ofthe rotor, the angle of the blade, the size of porosity of a filterattached to the periphery of the rotor, etc.

[0045] In the prior art, the milling of the carbon fiber has also beenperformed by means of the Henschel mixer, ball mill or mixing machine.This milling cannot be stated to be an appropriate procedure because notonly does pressure apply to the carbon fiber in the direction of thediameter thereof to thereby increase the probability of longitudinalcracks along the fiber axis but also the milling takes a prolongedperiod of time.

[0046] The above primary heat treatment prior to the milling may beperformed in an inert gas at 250 to 1500° C., preferably 400 to 1200°C., still preferably 600 to 1000° C.

[0047] In the carbon fiber derived from mesophase pitch, thecrystallization degree of the carbon is increased with the increase ofthe heat treating temperature, thereby growing the graphite layer, whoseplane is oriented parallel to the fiber axis. Thus, when heat treatmentis conducted in an inert gas at temperatures exceeding 1500° C. beforemilling, the carbon fiber is likely to suffer from cleavage and breakagealong the graphite layer plane having grown along the fiber axis. Theresultant milled carbon fiber is not desirable because the proportion ofreactive broken surface area to the total surface area of the milledcarbon fiber is high to thereby promote the reaction between thereactive carbon and the metal.

[0048] The milled mesophase-pitch-based infusibilized pitch fiberobtained by milling directly after the infusibilization or the milledprimarily heat-treated carbon fiber obtained by milling after theprimary heat treatment, is subjected to a high-temperature heattreatment at 1500° C. or higher, preferably 1700° C. or higher, stillpreferably 2000° C. or higher.

[0049] High-temperature heat treatment at temperatures lower than 1500°C. is not suitable because the degree of graphitization of the milledcarbon fiber is so low that the reaction with metals is likely to occur.

[0050] The high-temperature heat treatment after milling causes highlyreactive carbon exposed on the cut surface from the fiber interiorduring milling to undergo cyclization and thermal polycondensation, sothat the fiber cut surface can be converted to the state of lowreactivity.

EFFECT OF THE INVENTION

[0051] As described above, the milled carbon fibers of the presentinvention have a fiber cut surface and a fiber axis intersecting witheach other at cross angles, the smaller one thereof being at least 65°on the average. Thus, even when the graphite layer plane has achievedhigh-level growth, the above milled carbon fiber has low reactivity witha metal of high temperature or the like during the molding or usethereof because the proportion of reactive exposed surface of the innerportion of the fiber is small, so that the use of the milled carbonfiber can improve the mechanical strength and high-temperature heatresistance of the carbon fiber/metal composite material.

[0052] The process for producing milled carbon fibers according to thepresent invention comprises melt spinning of mesophase pitch,infusibilization, milling of the infusible pitch fibers as obtained orafter a primary heat treatment at 250 to 1500° C. in an inert gas, and ahigh-temperature heat treatment at 1500° C. or higher in an inert gas.Thus, not only can milled carbon fibers for metal reinforcement havinglow reactivity with a metal of high temperature or the like during themolding or use thereof so as to be suitable for improvement of themechanical strength and high-temperature heat resistance of thecomposite material be provided, but also the degree of graphitization ofthe carbon fiber can be regulated by selecting appropriate temperaturein the high-temperature heat treatment, so that materials suitable forintercalation into graphite layers or for application to fields wherethe crystallinity of the graphite is utilized can be obtained.

EXAMPLES

[0053] The present invention will further be illustrated with referenceto the following Examples, which should not be construed as limiting thescope of the invention.

Example 1

[0054] A starting material of optically anisotropic petroleum mesophasepitch having a softening point of 280° C. was melted and drawn through anozzle comprising a 3 mm wide slit and, arranged therein, a line of 1500spinning orifices each having a diameter of 0.2 mm while injecting hotair through the slit, thereby obtaining pitch fibers. The spinning wasconducted at a pitch discharge rate of 1500 g/min, a pitch temperatureof 340° C., a hot air temperature of 350° C. and a hot air pressure of0.2 kg/cm²G.

[0055] The spun pitch fibers were collected on a belt having acollection zone of 20-mesh stainless steel net while sucking fibercarrying air from the back of the belt.

[0056] The resultant collected fiber mat was heated in air whileelevating the temperature from room temperature to 300° C. at an averageheat-up rate of 6° C./min to thereby infusibilize the fiber mat.

[0057] Part of the thus obtained infusibilized mesophase-pitch-basedfibers were milled with the use of a cross flow mill to obtain milledinfusibilized fibers, which were successively graphitized at 2650° C. inargon.

[0058] An SEM observation of the thus obtained milled carbon fibersderived from mesophase pitch showed that the smaller cross angle of thefiber cut surface and the fiber axis intersecting with each other was87° on the average, and that the specific surface area of the milledcarbon fibers was 1.5 m²/g.

[0059] The average length of the milled carbon fibers was 750 μm.

[0060] The thus obtained milled carbon fibers and a powdery aluminumalloy containing 4.5 wt. % of magnesium were uniformly mixed in a weightratio of 25:75, and charged into a metal mold.

[0061] The charged mixture was held at 450° C. for 30 min, and hot-pressmolded under a pressure of 1000 kg/cm² for 20 min into a test specimenof 2 mm in thickness, 10 mm in width and 70 mm in length.

[0062] This test specimen was subjected to the 3-point bending testaccording to JIS (Japanese Industrial Standard) R7601, and the bendingstrength was determined to be 18 kg/mm².

[0063] Another test specimen was prepared in the same manner as above,heated at 600° C. for 5 hr, and subjected to the above bending test. Thebending strength was 17 kg/mm², which indicated that there wassubstantially no strength deterioration.

Example 2

[0064] Another part of the fibers infusibilized in Example 1 weresuccessively subjected to a primary heat treatment at 1250° C. innitrogen, milling and a high-temperature heat treatment at 2500° C. inargon.

[0065] The obtained milled carbon fibers had an average smaller crossangle of 82°, a specific surface area of 6.8 m²/g, and an average fiberlength of 700 μm.

[0066] A test specimen of fiber-reinforced aluminum alloy was preparedfrom the milled carbon fibers derived from mesophase pitch, and thebending test thereof was performed in the same manner as in Example 1.

[0067] The bending strengths measured immediately after molding andafter successive heating for the predetermined period were 17 kg/mm² and15 kg/mm², respectively.

Comparative Example 1

[0068] Still another part of the fibers infusibilized in Example 1 weresuccessively subjected to a high-temperature heat treatment at 2500° C.and milling. An SEM observation showed that many of the milled fiberssuffered from longitudinal cracks along the fiber axis, that the averagesmaller cross angle was 57°, and that the cut surfaces were markedlyuneven.

[0069] The milled fibers had a specific surface area of 12.3 m²/g and anaverage fiber length of 650 μm. The 3-point bending test was conductedin the same manner as in bending test was conducted in the same manneras in Examples 1 and 2. The bending strength immediately after the testspecimen molding was 15 kg/mm² which could stand comparison with thoseof the Examples. However, the bending strength after successive heatingat 600° C. was 7 kg/mm², which indicated an extreme deterioration of thebending strength.

What is claimed is:
 1. Milled carbon fibers produced from mesophasepitch, which have a fiber cut surface and a fiber axis intersecting witheach other at cross angles, the smaller one thereof being at least 65°on the average.
 2. The milled carbon fibers as claimed in claim 1 ,which have a specific surface area as measured by the BET method of 0.2to 10 m²/g.
 3. A process for producing milled carbon fibers, whichcomprises the steps of: melt spinning mesophase pitch to obtain pitchfibers; infusibilizing the obtained pitch fibers; milling the obtainedinfusibilized pitch fibers; and subjecting the obtained milled fibers toa high-temperature heat treatment at 1500° C. or higher in an inert gas.4. A process for producing milled carbon fibers, which comprises thesteps of: melt spinning mesophase pitch to obtain pitch fibers;infusibilizing the obtained pitch fibers; subjecting the obtainedinfusibilized pitch fibers to a primary heat treatment at 250 to 1500°C. in an inert gas, milling the resultant primarily heat-treated carbonfibers; and subjecting the obtained milled fibers to a high-temperatureheat treatment at 1500° C. or higher in an inert gas.